Python UserDict

In Python, UserDict is a class in the collections module that provides a wrapper around dictionary objects. It allows creating custom dictionary-like classes by extending UserDict and overriding its methods, making it useful for building specialized dictionaries with additional functionality or behavior.

Syntax

collections.UserDict([initialdata])

Parameters:

• initialdata: The UserDict constructor can take an optional dictionary or mapping object to initialize the data.

Return value:

Returns a dictionary-like object that can be subclassed to customize behavior.

Example 1: Subclassing UserDict

In this example, a custom class is created by subclassing UserDict and overriding common dictionary methods. This shows how to extend and customize dictionary behavior:

from collections import UserDict

class MyUserDict(UserDict):
  def __setitem__(self, key, value):
    print(f"Setting {key} = {value}")
    super().__setitem__(key, value)

  def __getitem__(self, key):
    print(f"Getting {key}")
    return super().__getitem__(key)

d = MyUserDict()
d["x"] = 42
print(d["x"])

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The output is:

Setting x = 42
Getting x
42

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Example 2: Basic Wrapper Usage

In this example, UserDict is used directly to wrap a dictionary. The .data attribute exposes the underlying dictionary object:

from collections import UserDict

d = UserDict({"a": 1})
d["b"] = 2
print(d)
print(d.data)

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The output is:

UserDict({'a': 1, 'b': 2})
{'a': 1, 'b': 2}

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Example 3: Logging on Set/Delete

In this example, dictionary modifications are logged by overriding __setitem__ and __delitem__:

from collections import UserDict

class LoggingDict(UserDict):
  def __setitem__(self, key, value):
    print(f"SET {key!r} = {value!r}")
    super().__setitem__(key, value)

  def __delitem__(self, key):
    print(f"DEL {key!r}")
    super().__delitem__(key)

d = LoggingDict()
d["name"] = "Aris"
del d["name"]

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The output is:

SET 'name' = 'Aris'
DEL 'name'

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Example 4: Case-Insensitive Keys

In this example, keys are treated case-insensitively by normalizing them to lowercase before storing or retrieving:

from collections import UserDict

class CaseInsensitiveDict(UserDict):
  def _key(self, key):
    return key.lower() if isinstance(key, str) else key

  def __setitem__(self, key, value):
    super().__setitem__(self._key(key), value)

  def __getitem__(self, key):
    return super().__getitem__(self._key(key))

d = CaseInsensitiveDict()
d["Name"] = "Mamta"
print(d["name"])
print(d["NAME"])

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The output is:

Mamta
Mamta

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Codebyte Example: Default-Like Behavior

In this example, a default factory is added, similar to collections.defaultdict(), so missing keys automatically get initialized:

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